[Caml-list] Insufficient polymorphism in local let.

[Caml-list] Insufficient polymorphism in local let.

[David Brown]

I'm trying to figure out how to get 'lmust' in 'get2' in the code
below to work.  It seems that it's type is being inferred as

   string -> string option -> string

instead of

   string -> 'a option -> 'a

The 'must' definition keeps it's polymorphism, as seen in 'get1'.  Is
there some way I can annotate the definition of 'lmust' so that it
will work?  Lifting 'lmust' out of 'get2' doesn't seem to help.

Thanks,
David

let must section name = function
   | Some o -> o
   | None -> 
     Printf.eprintf "Expecting %s in %s\n%!" name section;
     exit 1

type foo = { a: string; b: int }

let get1 a b =
   { a = must "get1" "a" a;
     b = must "get1" "b" b }

let get2 a b =
   (* This isn't polymorphic enough. *)
   let lmust = must "get2" in
   { a = lmust "a" a;
     b = lmust "b" b }

Re: [Caml-list] Insufficient polymorphism in local let.

[Gabriel Scherer]

This is a consequence of value restriction, with a direct workaround.
See the FAQ:

  "A function obtained through partial application is not polymorphic enough"
  http://caml.inria.fr/resources/doc/faq/core.en.html#eta-expansion

  let get1 a b =
    let must x = lmust "get2" x in
    ....

On Fri, Mar 23, 2012 at 5:58 AM, David Brown <caml@davidb.org> wrote:
> I'm trying to figure out how to get 'lmust' in 'get2' in the code
> below to work.  It seems that it's type is being inferred as
>
>  string -> string option -> string
>
> instead of
>
>  string -> 'a option -> 'a
>
> The 'must' definition keeps it's polymorphism, as seen in 'get1'.  Is
> there some way I can annotate the definition of 'lmust' so that it
> will work?  Lifting 'lmust' out of 'get2' doesn't seem to help.
>
> Thanks,
> David
>
> let must section name = function
>  | Some o -> o
>  | None ->     Printf.eprintf "Expecting %s in %s\n%!" name section;
>    exit 1
>
> type foo = { a: string; b: int }
>
> let get1 a b =
>  { a = must "get1" "a" a;
>    b = must "get1" "b" b }
>
> let get2 a b =
>  (* This isn't polymorphic enough. *)
>  let lmust = must "get2" in
>  { a = lmust "a" a;
>    b = lmust "b" b }
>
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